The development of cytotoxic lymphocytes in vitro decreases in DBA/2, C57B1/6 and NZB mice as a function of age. The decrease did not correlate with the onset of autoimmune disease in NZB but rather seemed to be a function of aging. The decreased capacity to generate cytotoxic lymphocytes in vitro from spleen cells of aging mice is associated with an increase in a demonstrable suppressor cell population.